Impedance tuners are often used for testing, tuning and calibration of electronic devices. Also, impedance tuners are the most common method for radio frequency (RF) and microwave (MW) amplifiers to be tested for measurement of performance. Impedance tuners can be used on load-pull and noise measurements at microwave and millimeter-wave frequencies.
An Impedance tuner includes a transmission line, such as a slabline, coaxial line, or waveguide. Placement of capacitive objects such as probes along the transmission line alters the impedance or electronic profile seen by the device under test (DUT) which is connected or coupled to the tuner transmission line. The object may be placed axially along the transmission line to affect the phase, while movement of the object transverse to the transmission line will alter impedance magnitude or gamma effects. In automated tuners, motors are used to position the capacitive objects along the transmission line and transverse to the transmission line.
Using a motor to repeat the positional movement along and transverse to the transmission line is important for accuracy. With frequencies in the gigahertz (GHz) range, even small errors in placement of the objects or probes can be very significant.
Today's manual tuners use high precision micrometers to measure the distance traveled along the transmission line but they still require a user interface for positioning and are limited by the precision of the micrometer. On some known automated tuners, a location (“home”) sensor is used as a reference start point and stepper motors are used to drive the object or probe along the transmission line axis and transverse to the transmission line. The stepper motor's complete rotation is divided into fractions similar to a pie. Each minor movement of the motor equals a slice of the pie. The motor stator includes wire coils that generate magnetic fields when electrically energized. The motor rotor typically also has magnets which respond to the magnetic fields. The magnetic field generated by the stator moves the rotor in segments of a full rotation. A stepper motor is driven by a series of electrical pulses, where each pulse causes the motor to rotate by the defined angle (a fraction of one full rotation). The amount moved can be easily calculated by counting the number of pulses that are sent. However, if the pulse produces insufficient current to move the motor such that the motor gets stuck and doesn't move, then the calculated position will be wrong.
The motor may be attached to a screw-like shaft, called a leadscrew, to propel a carriage. The carriage which supports the capacitive objects or probes travels along the screw-like shaft, by engagement with internal threads on the carriage. As the shaft is rotated by the motor the carriage moves in one direction. Reversing the motor will rotate the screw in the opposite direction, which moves the carriage in the opposite direction. Due to physical and material capabilities the cuts in the screw-like shaft (“threads”) typically do not match identically to the internal threads on the carriage. Thus an error in movement when reversing directions becomes evident.
Another common approach is to drive the carriage using a gear on a linear rack gear, as shown in FIG. 8. As with the screw-like drive, minor mismatches in the gears and other manufacturing limitations will produce some uncertainty about the exact location where the carriage will stop, even if the motor shaft repeated perfectly. This open loop control used in the past limits the positional accuracy that is possible.
Another error that may happen is there may be a limitation that prevents the carriage from moving. If this is to occur, the rotor of the motor will not move even though the signal to the stator has been sent. This error in position will affect all other position requirements afterwards.
Mechanical impedance tuners may have multiple motors. The limitation of positional accuracy described above applies to each motorized axis of an impedance tuner separately. FIG. 1 shows only one motor for simplicity of explanation, but the principle applies to any motorized component of an impedance tuner.
A common tuner configuration uses a carriage which moves parallel to the transmission line, and one or more motors mounted on that carriage to move capacitive objects transverse to the transmission line. A capacitive object mounted on the carriage moves parallel to the transmission line when the carriage moves, and is moved transverse to the transmission line by a separate motor mounted on the carriages with the capacitive object. This allows the capacitive object to move in two dimensions independently. In this case, the mass of the loaded carriage is much more than one capacitive object. The larger mass requires more motor force to move, and therefore may be more susceptible to stalling or not moving correctly for every pulse sent to the stepper motor.